In some methods for growing crystals, such as the Czochralski method, a seed crystal is brought into contact with the surface of a melt and then withdrawn from the melt. A crystal grows on the seed as it is withdrawn. The seed and growing crystal are sometimes also rotated about a vertical axis as they are withdrawn. Growth instabilities often occur in large crystals grown using this technique. As an example, the crystal may begin to grow in a spiral shape instead of a desired cylindrical shape. Growth instabilities may result in stresses due to variations in thermal expansion coefficients within the crystal, which may cause the crystal to shatter. This shattering is more likely when significant temperature gradients are present in the melt and in the atmosphere above the melt in which the crystal grows.
Growth instabilities may be caused by sufficiently large temperature gradients, accumulation of impurities in the melt, changes in the charge states of some of the melt constituents leading to creation of different molecular complexes, as well as by excesses of melt constituents accumulating at the interface between the surface of the melt and the growing crystal.
In the growth o f rare-earth oxyortho silicate scintillator crystals, oxygen vacancies in the crystal lattice may act as charge traps that lower the amount of charge carriers (electrons and holes) generated when the crystal absorbs ionizing radiation. The results are a lowering of scintillation efficiency and an undesirable persistent emission of light from the crystal known as afterglow. Reducing concentration of such oxygen vacancies is therefore desirable. This has been partially achieved by annealing the grown crystal in an oxygen-containing atmosphere. This is an extra step in the preparation of such crystals.